Electrical energy conversion systems adapted to produce electrical energy for direct introduction on the power grids starting from electrical energy generated, for example, from one or more renewable sources, such as photovoltaic and wind generation plants, are known.
The aforesaid electrical energy conversion systems usually comprise an inverter apparatus (commonly known as inverter) adapted to convert DC (Direct Current) input voltage into an AC (Alternating Current) output voltage having the amplitude and frequency features required by the power grid. These inverter apparatuses are adapted to be connected directly to the power grid so as to supply the electrical energy required by the various devices, such as, for example, the electrical household appliances present in a home, or to transfer the generated energy to the power grid.
Systems of this type are subject to problems related to possible transient phenomena, present on the power grid, which may momentarily alter the features of the power grid voltage.
Indeed, in a common inverter, connected to the power grid, which converts the energy generated by one or more photovoltaic, wind generation sources etc., the transient currents, for example, resulting from an overvoltage on the power grid, freely transit through the recirculation diodes anti-parallelly to the switching devices of the inverter. Usually, these recirculation diodes are not sufficiently robust to resist the high circulating transient currents, which can reach levels of hundreds or even thousands of amperes of amplitude, and are thus lethal for the aforesaid recirculation diodes.
Additionally, the switching devices themselves may need to switch these transient currents with a consequent high stress which very often leads to the destruction of the devices concerned by the described transient.
Accompanying FIGS. 1 and 2 show an example of the described situation.
FIGS. 1 and 2 show inverters of the single-stage and of the double-stage type for the direct connection to the power grid. In general, double-stage inverters may be provided with one or more independent input channels. When a transient overvoltage 11 occurs on the power grid 10, a transient current 12 of much higher frequency than that of the power grid freely crosses the recirculation diodes 13 of the inverter and the filter capacitor on the DC voltage from the photovoltaic assemblies. This current flows in a circuit of usually very low impedance and may reach even peak values of thousands of Amperes.
It is apparent that this current is very dangerous for the semiconductor devices present in the inverter, the switches and recirculation diodes in particular.
It is thus the object of the present invention to introduce a device adapted to solve the technical problem described above, and consequently to allow the semiconductor devices of the inverter, and thus the inverter itself, to survive the transient phenomena present on the power grid.